Electrodeposition of cadmium-tin alloy



Patented Sept. 2, 1952 ELECTBdDEPOSI TION OF ICADMIUMTlN ALLOY v Russell D. Gray,"Jr.,'Haw0rth, andWilliam 'Paecht, West Caldwell, N.;'J., assignors tor-Cur tiss-Wright Corporation, a corporation of Delaware No Drawing.

Abplication'Dece'mbe'r 1', 1948, Serial No. 62,965 I 6 Claims. (cram-43)- This invention relates to electrodeposition and is particularly directed to electrodeposition of a cadmium-tin alloy and/ or to articles plated with said alloy.

It is known that the corrosion, resistance of metal parts,particular1y parts made from ferrous metals, can be greatly improved by electrodepositing a surface layer of cadmium, 'zinc, or tin over said parts. Thus at least some of the ferrous metal parts of aircraft engines are generally provided with a coating of cadmium in order to increase the corrosion resistance of saidparts.

If a part to be provided with a corrosion resistant or preventive coating is designed for operation with only small clearanc relative to the thickness of said coating, then the thickness of the basis metal of said part must be made less than the overall thickness of said part by an amount approximately equal to the thickness of said coating. From the standpoint of the strength of said parts, therefore, the corrosion resistant coating deposited thereover should be as thin as possible. Even ifa part operates with a large clearance in space, it is desirable that its corrosion resistant coating be as thin-as possible since with a thin coating the part is lighter in weightand the coating is less expensive. Accordingly an object of this invention comprises the provision of a novel and simple method of plating' metal parts with a thin coating having the desired degree of corrosion resistance. I

Another object of the invention is to provide an electroplating process whereby a satisfactory corrosion resistant coating can be obtained over a wide range of such factors 'as bath temperature, current density, composition of bath, etc.

A still further object of the invention is to pro- Videan-eIectropIating process for plating metal parts with a corrosion resistant coating wherein said process has a cathode'efiiciency of approximately 100%. If the cathode efficiency is substantially less than 100%, considerable hydrogen is evolved at the cathode or parts bein plated and said hydrogen may become absorbed in said parts thereby rendering said parts brittle and lowering their fatigue resistance. This is particularly objectionable in the case of steelparts of high hardness. Additional objects of the invention will become apparent from the following detailed description.

Specifically the P esent invention comprises electroplating metal parts with a cadmium-tin alloy from an acid bath of' cadmium fluoborate and stannous fluoborate within which said parts are immersed. Such an electroplating bath is ,2 c, I hereinafter termed a cadmium-tin fluoborate bath. The metal partsto be plated are immersed in the bath and cadmium andtin electrodes are also immersed therein. A source of electric cur rentis, connected to said partsand: electrodes for current-flow through th .bath 'insuch .acdirection that said parts comprise cathodes andsaid electrodes comprise anodes whereupon cadmium and tin are deposited together .on; said parts as a cadmium-tin alloy.

An acid cadmium-tin fluoboratebath having 80 grams of cadmium, 20 gramsof tin perliterzof 'bath is preferred. A suitable bath is made from the-following constituents inthe quantities indie cated in the following table .with enough water to make a liter. 1

- (iontcenre 1011 Constituents if a S 01w Quantlty tion - percent Cadmium Fluoborate :Cd (13 F4)z 4 -51. 66 246.6 cc. Stannous Fluoborate SI1(BF4)2 43. 50 71.2 cc. Boric Acid ElsBOz 20 grams. Ammonium Fluohorate N H4BF 50 grams. Fluoboric Acid HBF4 42 .60 cc. Phenol Sulfonic Acid HOCGH4S03H 70 2.5 00. Processed Protein Powder 1-2 grams.

The cadmium fluoborate and stannous fluoborate solutions used in making the above describedbath have the decimal concentrations of 51.66% and 43.50% .respectively as indicated in the above table because'the solutions as purchased happened to have thesespecific concentrations. I

- Boric acid is .added to a bath to prevent cadmium or tin fluorides from precipitating-out and to provide better anodecorrosion. The fluoboric acid and ammonium fluoboratecomprisesbuffers, that is these constituents operate to prevent rapid changes in the pH value of the bath. In addition, ammonium hydroxide (NH4OH)" is added to decrease the acidity of..the:bath in order to bring its pH to the desired range of 3to 5. If the pH of the bath exceeds 5 fluoboric acid is added to lower the .pI-I to thedesired :range of 3.to 5. If the pH of the bath exceeds the limits of this desired range of 3 to'-5,-ca'dmium or tin-precipitate outof the bath.

Since boric acid and hydrofluoric acid (HF) react to produce fluoboric acid, they could be used in place of the fluoboric acidin making up the bath. Similarly since ammonia, hydrofluoricacid and boric acid react to produce ammonium fluoborate they could be-substituted-for the latter constituent in making up the bath. In addition since the ammonium hydroxide added to give the bath the required pH value reacts with the fluoboric acid in the bath to produce ammonium fluoborate it is not essential that this latter constituent be added to the bath as such.

The exact nature of the processed protein powder used in the above described bath is not known. However, this constituent is sold under included in the bath comprise addition agents which are effective to increase the density and fine grain structure of the alloy plated by the bath and in addition are effective to prevent rough build-up of said alloy at sharp corners or edges. The processed protein powder may also b used alone without the phenol-sulfonic acid. In addition, iniplace of the processed protein powder, animal bone glues with or without the ph'enol-sulfonic acid have been found satisfactory as addition agents in the above bath.

With the above described plating bath there are no rapid changes in the pH of the bath as the plating proceeds. At suitable intervals the pH of the bath can be checked in any conventionalmannen-for example by means of brom cresol green indicator or pH' papers. As already stated the pH of the bath can be adjusted to the desired range of 3 to 5 by adding ammonium hydroxide to increase its pH value and by adding fiuoboric acid to decrease its pH value.

The bath is preferably used at room temperature with a cathode current density of from 15 to 40 amperes per square foot of cathode area to be plated. In general this current density can be decreased below this range but this increases the plating time required. No agitation of the bath is necessary and this current density can be increased to as much as 100 amperes per square foot. If, however, the bath is agitated the current density may be increased to as much as 350 amperes per square foot. With a cathode current density of 30 amperes per square foot a cadmium-tin alloy surface layer of .000]. inch thickness is deposited in about 2 minutes.

The anodes preferably comprise separate cadmium and tin electrodes although an alloy of cadmium and tin may be used. Separate cadmium and tin anodes are preferred to anodes of a cadmium-tin alloy because of the superior corrosion in the plating bath of the separate cadmiumand tin anodes. In order that the cadmium and tin content of the above described plating bath remain'substantially unchanged as the plating proceeds, the immersed surface of the parts being plated (cathodes) should be substantially equal to that of the anodes. In addition the immersed surface of the tin anodes should beapproximately three times that of the cadmium anodes, that is, approximately 75% of the immersed anode surface area should be tin with the balance cadmium. Thus although the ratio of cadmium to tin in the bath (80 grams of cadmium to grams of tin) is greater than unity the ratio of the immersed surface area of the cadmium anode to the immersed surface area of the tin anode cadmium area to 75% tin area) preferably is less than unity. Obviously, however, maintenance of the original cadmium and tin content of the bath, although preferable, is not essential so that it is possible to use anodes of metals other than cadmium and/or tin.

During .the plating operation a'smut forms on theanodes- This smut is preferably kept out of the bath. For this purpose the anodes are enclosed in cloth bags of a material not attacked by the bath.

Tin is bivalent in an acid plating bath and is tetravalent in an alkaline bath so that when plating from a bath having a pH less than 7, the

rate at which tin is plated is doubled (for a given current density) as compared to the rate at which tin is plated from a bath having a pH greater than 7. Therefore the acid plating bath has a high cathode efiiciency and the quantity of hydrogen evolved at the cathode is reduced to a minimum. Actually the above described plating bath operates with a cathode efficiency of approximately l00%. As already explained this is important in order to avoid hydrogen 'embrittlement of hard steel parts being plated. Another advantage of an acid plating bath over an alkaline plating bath is its higher electrical conductivity with almost anode efiiciency. In addition the acid bath can be operatedat room temperature.

The .alloy coating or surface layer electrodeposited by said bath comprises 1'7 to 35% tin with the remainder principally cadmium. In general. within this range the corrosion'resistance of the alloy is better with the higher percentages of tin. The percent of tin in this alloy deposit increases with increase in the acidity of the bath and with increase in the density of the plating current. However an increase in the percent of tin in the alloy lowers its melting point. An alloy having approximately 30% tin and 70% cadmium has a melting point of approximately 475 F. The cadmium tin alloy electrodeposited by the bath of the present invention is dense, fine grained and extremely corrosion resistant.

Prior to the present invention, metal articles have been plated with a cadmium-tin alloy coating as for example by the process described in Patent No. 2,397,522. This prior art process uses a bath of an alkaline stannate and an alkaline cyanide of cadmium at elevated temperatures. The cathode efficiency of this prior art process is appreciably less than 100% so that hydrogen embrittlement of the parts being plated is likely to occur. Metal articles have also been provided with a cadmium-tin alloy coating by first plating a layer of tin and then plating a layer of cadmium over said tin layer. The metal article with its successively deposited layers of tin and cadmium is heat treated so as to diffuse the cadmium and tin to form an alloy of these metals. All these prior art processes are obviously more complicated than that of the present invention. Another prior art process for plating a cadmium-tin alloy is disclosed in Patent No. 2,093,031. This prior art process, like other prior art processes uses an alkaline plating bath and therefore-has a low cathode efiiciency. In addition thepercentage of tin in the alloy deposited is so small that the increase in corrosion resistance of said alloy over a cadmium plate is very small compared to the increase provided by the cadmium-tin alloy deposited by the process of the present invention. 1

For a given thickness of deposit, metal articles electroplated with a thin alloy deposit of cadmium and tin, by the method of this invention,

have corrosion resistant properties superior to those obtained by plating said articles with any known coating by any other known electroplating method. Thus metal parts have been provided with highly satisfactory corrosion resistant properties by electrodepositing on said parts, by the method of this invention, a cadmium-tin aloy surface layer having a thickness of only .0 01".

The plating bath herein described comprises a bath of cadmium fiuoborate and stannous fluoborate having a pH of 3 to 5 and having approximately 80 parts of cadmium to 20 parts of tin. Actually, however, by varying the quantities of cadmium fluoborate and stannous fluoborate, the proportions of cadmium tin in the bath can be varied widely thereby varying the proportions of cadmium and tin in the alloy deposited by thebath. Also the invention is obviously not limited to the particular buffers and addition agents mentioned.

While we have described our invention in detail in its present preferred embodiment, it will be obvious to those skilled in the art, after understanding our invention, that various changes and modifications may be made therein Without departing from the spirit or scope thereof. We aim in the appended claims to cover all such modifications.

We claim as our invention:

1. The method of electrodepositing a cadmiumtin alloy on a metallic body comprising the steps of immersing said body in a bath of cadmium fluoborate and stannous fluoborate having a pH of approximately 3 to 5 and having approximately 80 parts of cadmium to 20 parts of tin; immersing cadmium and tin electrodes in said bath, 75% of the surface area of said electrodes being tin with the balance cadmium; and connecting said body and electrodes to a source of electric current for current flow through said bath in a direction such that said electrodes constitute anodes and said body constitutes a cathode.

2. The method of electrodepositing a cadmiumtin alloy surface layer on a metallic body comprising the steps of immersing said body in an acid bath of cadmium fluoborate and stannous fluoborate in which the ratio of the number of parts of cadmium to the number of parts of tin in said bath is greater than unity; immersing anode electrode means of cadmium and tin in said bath such that the ratio of the immersed cadmium surface area of said electrode means to the immersed tin surface area of said electrode means is less than unity; and connecting said body and electrode means to a source of electric current for current flow through said bath between said body and electrode means in a direction such that said body constitutes a cathode.

3. The method of electrodepositing a cadmium-tin alloy surface layer on a metallic body comprising the steps of immersing said body in an acid bath of cadmium fiuoborate and stannous fluoborate having a pH of approximately 3 and having approximately 80 parts of cadmium to 20 parts of tin; immersing anode electrode means of cadmium and tin in said bath; and connecting said body and electrode means to a source of electric current for current flow through said bath between said body and electrode means in a direction such that said bodyrconstitutes a cathode.

4. The method of electrodepositing a cadmium-tin alloy surface layer on a metallic body comprising the steps of immersing said body in an acid bath of cadmium fluoborate and stannous fluoborate having approximately 80 parts of cadmium to 20 parts of tin; immersing anode electrode means of cadmium and tin in said bath such that approximately of the immersed surface area of said electrode means is tin with the balance cadmium; and connecting said body and electrode means to a source of electric current for current flow through said bath between said body and electrode means in a direction such that said body constitutes a cathode.

5. The method of electrodepositing a cadmium-tin alloy surface layer on a metallic body comprising the steps of immersing said body in an acid bath of cadmium fluoborate and stannous fiuoborate having a pH of approximately 3 and having approximately parts of cadmium to 20 parts of tin; immersing anode electrode means of cadmium and tin in said bath such that approximately 75% of the immersed surface area of said electrode means is tin with the balance cadmium; and connecting said body and electrode means to a source of electric current for current flow through said bath between said body and electrode means in a direction such that said body constitutes a cathode.

6. The method of electrodepositing a cadmium-tin alloy surface layer on a metallic body comprising the steps of immersing said body in an acid bath of cadmium fluoborate and stannous fiuoborate in which the pH value of the bath is approximately 3 and the ratio of the number of parts of cadmium to the number of parts of tin in said bath is greater than unity; immersing anode electrode means of cadmium and tin in said bath such that the ratio of the immersed cadmium surface area of said electrode means to the immersed tin surface area of said electrode means is less than unity; and connecting said body and electrode means to a source of electric current for current flow between said body and electrode means in a direction such that said body constitutes a cathode.

RUSSELL D. GRAY, JR. WILLIAM A. PAECHT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS chemical Society, Vol. 25 (1914), pp. 303, 327-331.

Carlson et al., Monthly Review of the American Electroplaters Society, Vol. 33 (March 1946) pp. 255-260. 

1. THE METHOD OF ELECTRODEPOSITING A CADMIUMTIN ALLOY ON A METALLIC BODY COMPRISING THE STEPS OF IMMERSING SAID BODY IN A BATH OF CADMIUM FLUOBORATE AND STANNOUS FLUOBORATE HAVING A PH OF APPROXIMATELY 3 TO 5 AND HAVING APPROXIMATELY 80 PARTS OF CADMIUM TO 20 PARTS OF TIN; IMMERSING CADMIUM AND TIN ELECTRODES IN SAID BATH, 75% OF THE SURFACE AREA OF SAID ELECTRODES BEING TIN WITH THE BALANCE CADMIUM; AND CONNECTING SAID BODY AND ELECTRODES TO A SOURCE OF ELECTRIC CURRENT FOR CURRENT FLOW THROUGH SAID BATH IN A DIRECTION SUCH THAT SAID ELECTRODES CONSTITUTE ANODES AND SAID BODY CONSTITUTES A CATHODE. 